Electric measuring instrument.



ETHOMSON & RLSHAND. l ELECTRIC MEASUE'ING INSTRUMENT.

APPL-IUATION FILED DE( J.12, 1906.. Patented June 27 2 SHEETS-SHEET l.

witnesses: 33 a /U f4 V3 Inventors y Ehhu Thomson,

' oqtty ,f frinnofrmo" MEASURINGVINSTRUMENT;

" species-'mn ofreces-Yeast. f rateneea iai-me at; i911;

p, -appiieatoa aldfnceaue 12,' masseria N. 347,495.

To- 'all who/m it" concern:

Beit knownthat we, ELrHU THOMSON and Roemer? Sri-Anims residing, respectively, at Swampscott andLLynn, county of-Essex, Statepf Illflas'sachusetts, have 'invented certain "rnewfrafndy useful l. Improvements in .Elecs tri'ollvleasuring Instruments, of which the followingfis af specification.' Our' invention relates to electrical measuring instrumentsfand particularly to instru;

. ,"mentsintended` for indicating the instan- 'taneous values of: current or electromotiveA force'in direct current circuits.

` Que object of Vour invention is to provide anfinstrument inwhich the moving element carrying v`th'epo-inter has a large angular deflectionf and fmoves :through equal angles i forequalchanges Fin the current passing through the instrument. In other words, ouraiin is to provide an instrument having a longand' uniform scale.

'Another object of our invention is to provider-for damping thev moving element so thatfitjshall be practically 4dead beat. .f

"A further object of our invention is to. render thev instrument-incapable of being affected'by changes in temperature or by mag"- netio elds i produced by neighboring `current-carryingrconductors or otherwise.V

' 'For the purpose of accomplishing the above and otherobjects, our invention comprises a magnetic system in which there are two .concentric annular fields and a currentcarrying'conductor supported for movement inthese ields A suitable damping device is attached to` the moving element of which the current-carrying Vconductor is ai part, and a1 specially designed spring furnishes the counter-torque.A -This spring for furnishing the counter-torque is not lclaimed in'this case,

but-"is 'made thesubject of another applicationfor Letters Patent, Serial ,'No. 347,484 filed Deel2, .1906, in the name of Elihu v 1 Thomson, oneof-thejoint inventors in' the 415 present'c'ase t l The details offconstructionand the mode of: operation of our improved mstrumentare clearlyjset forth the following speciication and illustrated in the accompanying drawings, `in whichi Figure 1 1s an elevation of the instrument,

part of the .dial being broken away to show the'interior-construction; Fig. 2 is a sectional plan on line .2-2 of Fig. 1; Fig. 3 shows the ring pole piece; Fig. f1 shows the plugl which fits the gap in the ring pole pieceillustrated in Fig. 3; Fig. 5 is a view in perspective of the moving element; Figs. 6, 7'

thefshaft; and Fig. 16 is a diagrammatic view o'f the spring applied to the shaft.

. As is'seen Jfrom Fig. 1 the casing 1, oit' the instrument is circular in form with the scale over which the pointer moves extending around approximately 2800 of its circumference. The casing consists of two parts: the base 1, preferablymade of cast iron, and the lcylindrical casing 2, preferably made of sheet met-al. On the base 1 are supported the working parts of the instiaiment The magnetic system maylconsist of two permanent magnets 3 of a shape somewhat resembling the letter C and arranged as shown i-n Fig.,

1 with their ends adjacent one another. To

vthe adjacent ends of these magnets 3 are fastened the pole pieces l and .5 by means of bolts 6 and 7, as shown 1n Fig. 1 or in any other suitable manner. Pole piece 4 (see Fig. 3) is extended to form a ring 8; this .ring pole piece being cut through at 9v for the purpose Vhereinafter explained.v A

plug 10, shown in Fig. Ll, is adapted to itv this-gap 9and make what is in effect a continuousfring. Pole piece 5, shown in Fig. 7,

has extending-therefrom an ear 11 which supports a center pole extension 12 (see Fig. 8) and a shell 13 .(see Fig. 6).rk The shell 13 is fastened to the pole piece by the center pole 12, the end of'which is threaded and passes throughthe hole 1/1 in the shell 13 and the hole in the ear 11, a nut 16 (see Fig. 2) serving to draw the shoulder 13EL on the center pole against the shell and thus hold the three parts together. It is desirable', in order to' produce a uniform distribution of the magnetic field, that the magnetic con# nection of the shell 13 with the pole piece 5 be made through the ear 11, which extends from the pole piece 5 a distance greater than the outside radius of the shell 13-,s0 that when the parts' are assembled, the shell 13 is not large enough to extend from the hole 15 in the ear-11 to the pole piece 5, and a small air gap exists between the outer sur face of the shell 13 and the body of the pole piece 5 when these parts have been bolted together by the center pole `12. With this construction the ringpole piece -is sandwiched between the center pole 12 and the shell 13 of the other pole piece, thus produ@ ing two perfectly uniform concentric an- 'nular fields.

The moving element of the instrument is supported on theshaft 17 mounted in jeweled bearings. The inner bearing 18 is supported in the center pole 12, the jewel being held in a plug which is screwed into 4a threaded hole in the center pole as is shown in Fig. 2. The outer` bearing 19 is supported in a bracket 20 'mounted on the pole piece 5 in any suitable manner las by` screws 21. On the shaft 17 is mounte'd a drum or thimble 2 2 made of copper, aluminum, or some other' non-magnetic `conductor. This thimble 22, which extends over the center pole 12, cuts the inner annular field and produces the necessary damping for the moving element. Fastened to the thimble 22 or in any other suitable manner supported to move with the shaft 17 is a bobbin 23 upon which is wound a coil-of fine insulated wire 21 (see Fig. 5). The opening in the bobbin 23 is large enough to allow it to move over the ring pole piece 8 without coming in contact therewith. The gap 9 in the ring pole piece allows the bobbin to be slipped over the ring after which the plug 10 can be inserted in the `gap and fastened there in any suitable manner as by being wedged therein. With this constructionit is seen that the current carrying coil 24: is so arra'nged that the outer and inner 'sides thereof are each bathed in a strong and uniform annular iield, and that, when a current is passedf through Vthis coil, the coil will tend 'to moyef in these fields, thereby producing'rotation of the moving element of which the coil is a part. In order to increase the damping j effect of the thimble 22, a projection 25 is formed on the inside of the plug 10, which has. the effect of reducing the air gap at that point between the ring pole piece 8 and the center pole 12, thereby increasing the density of the magnetic iiuX at this point. As the bobbin 23 doesvnot at any time pass through this part of the annular field, it is obvious that the air gap here may be made only large enough to allow the thimble 22 to .move freely therein.

In a long range measuring instrument of the kind described tthe spring which furnishes the counter-torque against which the tendency of the coil to move is exerted, must contain many turns in order to give satisfactory resiliency. The ordinary form of volute spring, if made very long, is liable to sag and is open to the rfurther Objection of changing its length with variations in the temperature. Such a spring, if used in an instrument, is objectionable as tending to disturb the pointer from its Zero position. We employ, therefore, a spring especially designed for use in along range instrument or elsewhere where it is desired to return a rotary member toits initial position after rotation through a considerable angle. 'As' previously stated this spring is made the subject of a separate applicationv for Letters Patent, but it will, nevertheless,

'requirements which make a long spring necessary without at the same time being -open to the Aobjections which 4stand in the way of the use of long volutes. With this arrangement it is seen that as' the shaft rotates one of the -volutes has a tendency to coil up and the other to uncoil, a result which is illustrated Fig. 15. Owing to t-he moderate lengt-h of the volutes there is no sagging, and a's they are wound in the same direction with their outer ends free to move, any expansion or contraction due to changes in temperature will result simply in a movement o-f the outer ends of the volutes wherethey are Ijoined together and will not affect the position of the shaft'to which thel'spring is attached. To permit Yinner end of one of the volutes is fastened.

is held by the arm 29 whichis adjustable on an aXis coinciding with the aXis of the moving' element.y The arm l29 is held by the frictional contact of the nut 30 which may 'y geen throughaaetoaea .tires K inneindsof'which areconnected withffinelsivlreswlnch extend along the shaft 17 from the coil 24, theeouterjzends i. Iof the coiled wires 31fand32 beingfastened to the postsfand34 mountedfonv the bracket 2O which supports the outside bearing of the moving element.

The pointer 35 fixed to move with the Shaft 17 extends outwardly so that its end passes over the dial 36 of the instrument.I As a protection to the working parts of the instrument the circular box 37, preferably of non-magnetic material, as aluminum or brass, is fastened to the base of the instrument and surrounds the whole interior mechanism, holes being cut at opposite sides in this box for the insertion of the pole pieces. The dial 'upon which the scale of the instrument is printed is preferably made of some translucent material and supported from the base-of the instrument by legs, one of which may be seen in plan view .at 38. Incandescent lamps 39 and 40 may be located behind the dial for illuminating it. The casing 2, which may be fastened to the base of -the instrument by thumb screws 41, or in any other suitable manner, carries a glass front 42 through which .the scale may be seen. This glass front is preferably arranged to carryat its center a disk 43 preferably made of sheet metal which conceals the inner mechanism of the instrument. The base of the instrument carries the binding posts 44 which are connected by the conductors 45 and 46 to the terminals 33 andv34 of the moving coil. In case the instrument is t-'o be used as a voltmeter an internal re-v sistance is interposed in conductors 45 or 46. If the instrument is to be used as an amnieter-the terminals of the instrument are connected across a shunt of low resistance in the manner well understood.

The instrument which we have shown and described is but one ofthe many forms in which our invention may be embodied. Its details of construction and proportion of parts may be varied Without departing' from the principles upon which our instrument is based. For example, Figs. 9, lO and 11 and Fig. l2 show modifications of the ring pole piece. In the formy shown in Figs. 9 t-o 11, the opening in the ring for the purpose of admitting the moving coil is made. at the point opposite the isthmus of iro'n which connects the ring withv the body of the pole piece. Withthis forniv of ring pole piece 47 a removable fpiecepof irong48is tted to the gap in thepole piece and may be fas-` tened therein by means of screws 49, or in any other suitable manner.` I n order to increase the damping. efect a small projecting theegapafzer t-hcradmissionof.the maring- -fcoil Y assay nerffa. V the project-inn25.on'theiplug 4 is made-by; drilling., out. la portionff,

plug 52 .whichlmay be held in place in any suitable manner as by screws, wedging, ory otherwise. A projecting piece 53 is secured on the inside of the ring in the same manner as in the form of pole piece shown in Fig. 9. .Other modiications in the construction of an instrument of this type will occur to those skilled in the art. It isour intention to includev all such in the following claims.

What we claim as new, and desire to secure by Letters Patent of the United States, is,--

1 In an electrical measuring instrument, a magnet, pole pieces on said magnet arranged to produce two concentric annular fields, a moving element comprising a damping cylinder mounted to rotate in one of the annular fields, a current-carrying coil mounted on said cylinder to cut the flux in both of the annular fields, and means for furnishing the counter-torque for the moving element.

magnet, a pole piece in the form ofa ring attached thereto, a second 'pole piece having two' concentric cylinders arranged one within the other, said ring pole piece being located between them, va moving element comprising a damping cylinder supported for rotation between the inner pole piece and the ring on an axis concentric with these parts, a current-carrying coil encircling said ring and carried by said damping cylinder, and means for furnishing the countertorque for the moving element.

p' 3. In an electrical measuring instrument, a magnet, 'a pole piece therefor in the form of a 'ring having a removable portion, a second pole piece having two concentric cylinders arranged one within the other, said ring pole piece'being located between them, a moving element comprising a damping cylinder supported for rotation between the inner pole .piece and the .ring on an axis concentric with these parts, a current-carrying coil 4encircling said ring and carried by said damping cylinder, and means for furnishing the counter-torquefor the moving element.

l 4. In an electricalpmeasuring instrument, a magnet, a pole piece therefor in the form ofa ring, a second pole piecev Vhaving two concentric' cylinders arranged one within said ring and the other surrounding it, Va moving element comprising a4 damping cylinder supported' for rotation between the jecting block of iron fastened on the inside o'f'f's'aidring to lessen the air gap at that point, a current-carrying coil encrclin said "ring andcarried by said damping cylinder,

5 an meansvfor furnishing the counter-torque for the moving element. l

5. An electrical measuring instrument comprising means for producin a magnetic field of force, a measuring co1 mounted 10 to turn therein, and a damping conductor movable therewith, the air gap being smaller p at ,a point outside the angle' of movement of the coil to increase the damping eiect.

In witness whereof, we have hereunto set our hands this eighth day of December, 1906. 15

ELII-IU THOMSON. ROBERT SHAND. Witnesses:

JOHN A. McMANUs, Jr., HENRY O. WESTENDARP. 

